linux-kernel - Re: [PATCH v10 2/3] KVM: s390: guest support for topology function

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Message-ID: <851dd253-8412-ed5f-3a97-980b3a3850cc@linux.ibm.com>
Date:   Tue, 28 Jun 2022 14:18:12 +0200
From:   Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.ibm.com, cohuck@...hat.com,
        david@...hat.com, thuth@...hat.com, imbrenda@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com, wintera@...ux.ibm.com,
        seiden@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v10 2/3] KVM: s390: guest support for topology function

On 6/28/22 12:58, Pierre Morel wrote:
> 
> 
> On 6/28/22 10:59, Janis Schoetterl-Glausch wrote:
>> On 6/20/22 14:54, Pierre Morel wrote:
>>> We report a topology change to the guest for any CPU hotplug.
>>>
>>> The reporting to the guest is done using the Multiprocessor
>>> Topology-Change-Report (MTCR) bit of the utility entry in the guest's
>>> SCA which will be cleared during the interpretation of PTF.
>>>
>>> On every vCPU creation we set the MCTR bit to let the guest know the
>>> next time he uses the PTF with command 2 instruction that the
>>> topology changed and that he should use the STSI(15.1.x) instruction
>>> to get the topology details.
>>>
>>> STSI(15.1.x) gives information on the CPU configuration topology.
>>> Let's accept the interception of STSI with the function code 15 and
>>> let the userland part of the hypervisor handle it when userland
>>> support the CPU Topology facility.
>>>
>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>> ---
>>>   arch/s390/include/asm/kvm_host.h | 11 ++++++++---
>>>   arch/s390/kvm/kvm-s390.c         | 27 ++++++++++++++++++++++++++-
>>>   arch/s390/kvm/priv.c             | 15 +++++++++++----
>>>   arch/s390/kvm/vsie.c             |  3 +++
>>>   4 files changed, 48 insertions(+), 8 deletions(-)
>>>
>> [...]
>>
>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>> index 8fcb56141689..95b96019ca8e 100644
>>> --- a/arch/s390/kvm/kvm-s390.c
>>> +++ b/arch/s390/kvm/kvm-s390.c
>>> @@ -1691,6 +1691,25 @@ static int kvm_s390_get_cpu_model(struct kvm *kvm, struct kvm_device_attr *attr)
>>>       return ret;
>>>   }
>>>
>>> +/**
>>> + * kvm_s390_sca_set_mtcr
>>
>> I wonder if there is a better name, kvm_s390_report_topology_change maybe?
>>
>>> + * @kvm: guest KVM description
>>> + *
>>> + * Is only relevant if the topology facility is present,
>>> + * the caller should check KVM facility 11
>>> + *
>>> + * Updates the Multiprocessor Topology-Change-Report to signal
>>> + * the guest with a topology change.
>>> + */
>>> +static void kvm_s390_sca_set_mtcr(struct kvm *kvm)
>>> +{
>>
>> Do we need a sca_lock read_section here? If we don't why not?
>> Did not see one up the stack, but I might have overlooked something.
> 
> Yes we do.
> As I said about your well justified comment in a previous mail, ipte_lock is not the right thing to use here and I will replace with an inter locked update.

Not sure I'm understanding you right, you're saying we need both? i.e.:

struct bsca_block *sca;

read_lock(&vcpu->kvm->arch.sca_lock);
sca = kvm->arch.sca;
atomic_or(SCA_UTILITY_MTCR, &sca->utility);
read_unlock(&vcpu->kvm->arch.sca_lock);

Obviously you would need to change the definition of the utility field and could not use a bit field like Janosch
suggested, unless you want to use a cmpxchg loop.
It's a bit ugly that utility is a two byte value.
Maybe there is a nicer way to set that bit, OR (OI, OIY) seem appropriate, but I don't know if they have a nice
abstraction in Linux or if you'd need inline asm.
> 
>>
>>> +    struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */
>>> +
>>> +    ipte_lock(kvm);
>>> +    sca->utility |= SCA_UTILITY_MTCR;
>>> +    ipte_unlock(kvm);
>>> +}
>>> +
>>
>> [...]
>>
>